Current engine designs such as those used in automotive and many other applications typically use an in-line or “V” arrangement of multiple pistons. Each intake and exhaust engine valve is opened and closed by a cam on one or more camshafts. Since each piston employs two or more valves, many cams are required for the engine. The camshafts are aligned parallel to the crankshaft that controls the oscillatory motion of the pistons. They extend approximately the entire length of the engine. The possible locations of the camshafts are limited due to the need to arrange them in concert with the valves they operate.
The arrangement of the camshafts is determined by the mechanical link between the cam and the valve the cam operates. The cam may operate directly on the valve, use a “push rod” and/or a “valve lifter” as a direct link between the cam and valve, or incorporate rocker arms to transfer the cam motion to the valve. A push rod is a small shaft that allows the cam and camshaft to be located some distance from the valve. A valve lifter is a small device that uses engine oil to maintain proper adjustment of the valve actuation mechanism. All of these options for valve actuation limit the practical locations for the cams and camshafts on the engine.
The rotation of each cam on each camshaft must be synchronized with the crankshaft in order for the valve to open and close at the proper position of the piston in the engine cycle. In four-stroke engines, the rotation of the camshaft is one-half as fast as the rotation of the crankshaft. Synchronization of the camshafts and crankshaft is accomplished in newer engines using timing belts or timing chains since the camshafts are often located too far from the crankshaft (for example, in the cylinder head) for them to be synchronized by gear arrangements alone. Timing belts can fail due to wear or breakage and require expensive replacement at specified intervals of operation.